Liquid crystalline phases (mesophases) are partially ordered intermediate phases existing between the crystalline solid and isotropic liquid Materials in a liquid crystalline phase can flow like liquids, while retaining several features of crystalline solids such as optical and electromagnetic anisotropy characteristics. These properties are due to a specific amount of positional or orientational order in their structure. Mesogens or mesogenic groups are chemical moieties that induce mesophases under certain conditions. According to the ways to generate a liquid crystalline phase, these groups can be classified as lyotropic (exihibits liquid crystalline phase in solution) and thermotropic (exhibits liquid crystalline phase in melt, a single component system) liquid crystals.
The two main types of liquid crystalline phases are the nematic and smectic mesophases. In nematic phases the molecules have only an orientational order, while in smectic phases they have both orientational and positional order in one or more dimensions.
When a thermotropic LC compound is heated, the solid changes into a rather turbid liquid at the melting point. The fluidity may be high for a nematic phase and relatively low for the smectic phases. When observed between crossed polarizers under a microscope, the fluid is found to be strongly birefringent. Upon further heating, another transition point is reached where the turbid liquid becomes isotropic and consequently optically clear (clearing point). Between these two transition points, the liquid crystal phase is thermodynamically stable Both phase transitions are first order and the latent heat at the clearing point is usually an order of magnitude smaller than the melting point.
The polarizing m is a classical and useful tool for the study of liquid crystals. Dependent upon the boundary conditions and the type of LC phase, specific textures are observed and used to classify the different phases.
Liquid crystal polymers were discovered in the 1950s, when Onsager and Flory theoretically predicted that rigid rod-like macromolecules should display liquid crystalline properties. An axial ratio of 6.42 is enough for a polymer to form an LC melt. However, the molecular weight must be high to achieve good mechanical properties. The first main chain thermotropic liquid crystalline polymer was reported by Roviello and Sirigu in the 1970s, and since then many patents have been published and several LC polymers were commercialized.
Compared to monomer liquid crystals, polymer liquid crystals can display similar behaviors, and be classified into thermotropic and lyotropic LCPs. Several well known classes of polymers including polyesters, polyethers and polyamides can exhibit liquid crystalline phases. According to different mesogen positions in the polymer, LC polymers can be classified as main chain, side chain and combined liquid crystal polymers. More complex architectures are also possible.
LCPs are quite different from the conventional polymers. They have properties that include low melt viscosity, fast cycle time in molding, very low mold shrinkage, excellent mechanical properties, solvent resistance, excellent barrier properties, low water absorption, low thermal expansion coefficient, excellent thermostability, low flammability, etc. Therefore, they have been explored for numerous applications in the following areas: high-strength and high-modulus fibers, precision molded small components, films exhibiting excellent barrier properties, novel composites, processing aids in the melt, reversible information storage, electro-optical displays and non-linear optical devices.
The mesogenic groups in LCPs are usually rod-like or disk-like molecules, such as two or more rigid cyclic units. Aromatic rings are the most common units used in liquid crystal polymer to provide rigid rod structures. The synthesis, structure, rheology, processing, performance and applications of many LCPs have been comprehensively described in the literature, including Demus, D., et al, Physical Properties of Liquid Crystals; Wiley-VCH Verlag GmbH: Weinheim, 1999; Kwolek, S. L. Encycl. Polym. Sci. Eng. 1987, 9, 1–61; Collyer, A. A.; Editor. Liquid Crystal Polymers. From Structures to Applications; Elsevier: London, 1992; Ciferri, A.; Krigbaum, W. R.; Meyer, R. B.; Editor. Polymer Liquid Crystals; Academic Press: New York, N.Y., 1982; and Isayev, A. I.; Kyu, T.; Cheng, S. Z. D.; Editors. Liquid-Crystalline Polymer Systems: Technological Advances. (Symposium at the 209th National Meeting of the American Chemical Society, Anaheim, Calif., Apr. 2–7, 1995.) [In: ACS Symp. Ser., 1996; 632]; ACS: Washington, D.C., 1996.
Thermotropic main chain liquid crystal polymers are the most important group of LCPs. They consist of mesogenic groups incorporated into the backbone of the polymer chain, and when prepared without flexible spacers, are usually known as wholly aromatic thermotropic LCPs. Because of their main chain stiffness and high packing density, they can exhibit excellent mechanical properties and are extremely useful in high-strength and high-modulus fibers. Since they form LC phases when melted, the viscosity in the melt state is relatively low, thus make the processing easy. Furthermore, the rod-like mesogenic groups can be aligned during the extruding or spinning process and give very high strength along the fiber direction.
Polyesters are a very important group of this class of polymers. Structures of some commercially important theremotropic copolyesters are listed in Table 1.
TABLE 1Structures of some thermotropic co-polyestersChemical StructureMonomers1p-hydroxybenzoicacid (HBA) 24,4′-biphenol (BP)/Terephthalic acid(TA) 36-hydroxy-2-naphthoic acid(HNA)/HBA 42-methylhydroquinone (2-MHQ)/TA 5Isophthalic acid (IA)/HBA/BP/TA
Generally, wholly aromatic thermotropic polyesters have poor solubility in normal organic solvents. Good solvents for this class of polymers include fluorinated compounds, such as pentafluorophenol (PFP), p-fluorophenol, trifluoroacetic acid, etc. Due to the poor solubility in common solvents, GPC data are usually not available in the literature. However, Kinugawa, et al. have investigated the molecular weight distributions of LC aromatic polyesters by the GPC-low-angle laser light scattering technique.
General characterization methods for this class of polymers include differential scanning calorimetry (DSC), polarized light microscopy, and wide-angle X-ray diffraction.
The ability to show anisotropy and readily induce orientation in the liquid crystalline state leads to materials with great strength in the direction of orientation, and thus, these polymers have received considerable attention as high-performance fibers, films and plastics, especially for injection molding applications.
The concept of a melt processable LC polymer is a natural extension of the discovery of KEVLAR® at DuPont, which is a wholly aromatic LC polyamide spun from concentrated sulfuric acid. Ekkcel I-2000 (copolyester from p-hydroxybenzoic acid (HBA), terephtalic acid (TA) and 4,4′-bisphenol (BP)) was the first melt spinable LC polyester reported in 1972. It has a melting point around 400° C., which is still too high for common melt spinning equipment.
In the 1970's and 1980's, aromatic LC polyesters were developed quickly and many LC polyesters were commercialized during this period. XYDAR® was first commercialized by Dartco Manufacturing Company in 1984 and was later manufactured by Amoco Chemical Company. It exhibits a melting point above 300° C. The VECTRA® family of LCPs was introduced by Celanese in 1985, with a melting point of 250–280° C.
Since these types of polymers offer a unique combination of properties, they are expected to offer potential solutions to problems which conventional materials are unable to solve. Currently, industrial activities are mainly concentrated on main chain thermotropic LCPs for injection molding applications.
Homopolymers from HBA or 6-hydroxy-2-naphthoic acid (HNA) exhibit high crystallinity and high melting point (higher than 600° C.). Although they provide excellent mechanical and thermal properties, their high melting points make them intractable and impractical for any commercial applications, since they are not melt spinnable or injection moldable. Thus, research has focused on developing new polyesters that have better tractability (lower melting point) without sacrificing other desirable properties.
The most common way to achieve this is to disrupt the regular chain structure. Until now, several methods were found to be effective in lowering the melting point of LC polyesters, such as the introduction of aliphatic spacer units on the backbone, using monomers with bent structures (kinks), ring substitution, “swivel” structures, and parallel-offset structures (crankshaft) into the backbone. However, a need for additional LCP having a desired balance of properties, including Tg, melting point (Tm), tensile strength and/or thermal stability, still exists.
Introducing aliphatic structures can give the backbone more flexibility, which disturbs the packing of the polymer chain and lowers the melting point. Numerous efforts have been made in the LC polyester area using this strategy. One example is X7G. By introducing the PET structure into the polymer, the melting point was lowered to about 230–300° C. Another example is SIVERAS, an LC polyester based on PET, introduced by Toray Industries, Inc. in 1994. It is melt spinable at 310–320° C.
The major drawback for this strategy is that the aliphatic structure also decreases the degree of liquid crystallinity and lowers the thermal stability and mechanical properties dramatically. The properties are decreased in proportion with the length of the flexible spacer and its content in the polymer.
Instead of using para-substituted monomers, meta- or ortho-substitution on the phenyl ring will introduce a bent structure into the backbone, thus disturbing the packing and lowering the melting point. An example of this is Ekonol, which is composed of units derived from monomers HBA, TA, BP and a small amount of isophthalic acid (IA). The polymer exhibits very high tensile modulus and strength as a fiber. The problem with this strategy is that the kink structure can not exceed a specific amount in the total composition, without loss of LC properties. It was reported that for kink units having a 120′ core angle such as isophthalic acid, the polymers will not exhibit liquid crystallinity with more than 60 mol % of kink units of the acids. For kink units having a 60′ core angle, the maximum ratio is 30–40 mol %. As the amount of the kinking component increased, the liquid crystallinity and the orientability of the polyesters from the melt decreased. Therefore, the level of tensile and flexural properties decreased. Very high plastic and tensile properties were only possible when the kink component was less than 10 mol %.
Introducing a substituent in the aromatic ring can cause a decrease in crystallinity and hence a drop of the melting point of the polyester. The substituents, especially asymmetrical substituents, can disturb the packing of the chain by inter-chain separation and by the random arrangements called internal copolymerization effect.
Different substituents, including halogens (Cl and Br), methyl, phenyl, and phenoxy, have been investigated for their effects on lowering the melting point. The size, the additional degrees of rotational and conformational freedom of the substituent has a great effect on how much the melting point can be lowered. This approach can also result in complete loss of LC behavior. If the percentage of the substituent is too high, this may disturb the packing and the polymer may lose all LC properties in the melt.
The “swivel” structure is shown below. Since the two phenylene rings are not in the same plane, they are twisted at a small angle with respect to each other, and the packing density of the polymer is lowered. The linkage “X” can be a direct bond, S, O, etc. Since the disturbing influence is along the backbone axis, the risk of losing LC properties is normally high, except in the case of a direct bond. This is due in part to the “kink” which is imparted by the O or S bond.

The simplest “swivel” structure is biphenol (BP), in which there is a direct linkage between the two rings. The liquid crystallinity of the polymers will not be completely lost even at 100 mol % of BP of the diols. The small twist angle of BP does have an effect on lowering the melting point. For example, Ekkcel I-2000 in which BP is one of the co-monomers, the melting point is more than 200° C. lower than homopolymer of HBA.
The common monomer used in this strategy is 6-hydroxy-2-naphthoic acid (HNA). The 2,6-naphthalene ring structure introduces a crankshaft structure in the polymer chain. After this modification, the melting points are lowered without sacrificing significant crystallinity since the backbone is still parallel to the original axis. Therefore, the LC properties and mechanical properties can be maintained even with a relatively high percentage of HNA monomer.
One of the most prominent high performance LCP polyesters developed was VECTRA®, derived from HNA and HBA. It is melt processable with common processing equipment capable of handling materials with melting points at 250–280° C. The excellent properties of VECTRA® polymers make them useful in a variety of applications such as optical fiber cables, fishing line and high strength fiber reinforced composites, etc.
From the discussion above, we can see that the introduction of “swivel” and “crankshaft” structures into the backbone of LC polymers are two of the best strategies to achieve low melting point for main chain LC polyester while maintaining excellent mechanical and thermal properties. Therefore, in order to obtain even better tractability and excellent mechanical properties, and investigate their structure-property relationships, wholly aromatic LC polyesters containing a phenylene-naphthalene structure would be desirable.

Although some compounds containing the phenylene-naphthalene structure have been reported, no polymers containing this subunit have been described in the scientific or patent literature. 2-(4-Hydroxyphenyl)naphthalene-6-carboxylic acid is disclosed in U.S. Pat. Nos. 5,151,549 and 5,146,025, but no description of any polymers prepare from the monomer appear in either patent.
Phenylene-naphthalene monomers are useful monomers for themotropic LC polyesters, as they may introduce additional dissymmetry into their monomers and polymers, combine the “crankshaft” and “swivel” effects together, and maintain wholly aromatic backbone structure. Therefore, better tractability can be achieved without sacrificing mechanical and liquid crystal properties.